Unearthly Earthstars by Paul Sterry

Earthstars are among the most bizarre fungi in Britain and most people’s reaction to a first encounter is that they are not of this planet. Their structure and appearance is odd enough, but with a bit of coaxing a few species will even change shape in front of your very eyes.

Above: The Collared Earthstar Geastrum triplex is the most widespread and familiar of its kind in Britain.

Earthstars make their first appearance above ground as spherical, puffball like structures. Although it is hard to generalise, in most species the outer layer then begins to split: at first resembling an unpeeled onion bulb. The splits become more apparent and soon the radiating arms expose an inner spore sac. The aim is to raise the spore sac above the surface of the ground, or leaf-litter, depending on where it is growing.

In earthstars of the genus Geastrum there is a single exit hole in the sac, through which spores are released. Water plays a vital role here because typically falling rain drops will be enough to trigger a puff of spores to be released.

Above: Raindrops will trigger spore release when they hit the spore sac of species such as this Collared Earthstar Geastrum triplex.

 

Above: The Arched Earthstar Geastrum fornicatum. The specific element of its scientific name relates the shape of its raised rays, which bear a fanciful resemblance to a Roman  ‘fornicatum’ or vaulted arch.

Water, and the lack of it, has another important role to play with a few species: in dry conditions the radiating arms fold back protect the shrivelled spore sac. But when rain falls something faintly miraculous happens – the arms reflex and the spore sac inflates ready to release spores when primed by a falling raindrop. The biological process of absorbing water from the atmosphere is termed hygroscopy and the Barometer Earthstar is a prime example.

 

Above: As its names suggests, the Barometer Earthstar Astraeus hygrometricus is hygroscopic – seen dried and shrivelled on the left, and rehydrated on the right.

With summer round the corner, it might not seem like the best time of year to look for earthstars. However, fortunately for naturalists they are resilient fungi and weathered specimens will last for several years. And hygroscopic species can provide the curious naturalist with a bit of fun – if you find what looks like a shrivelled onion, drip some water onto it and sit back and watch in amazement: in the space of a few minutes, it will return to life.

This speeded up time-lapse sequence shows ten minutes in the life of a rehydrating Weathered Earthstar Geastrum corollinum. This particular specimen is a least five years old and still performs its party trick in response to being splashed with a couple of drops of water.

 

Above: There are still discoveries to be made in the field of earthstars and Geastrum britannicum was confirmed by DNA analysis as being new to Britain in 2015.

 

Above: The Pepper Pot Earthstar Myriostoma coliforme looks more like a stage prop from a B Movie than a fungus. Instead of having a single release port for its spores, it has several.

 

Above: The tiny Cannonball Fungus Sphaerobolus stellatus is a bizarre relative of larger earthstar species whose fruit bodies appear on the surface of dung and organic mulch in flower beds. When mature, the fruit bodies split revealing a spore-containing mass inside. Osmotic pressure builds up under a membrane below the spore sac and sooner or later the mass is ejected at speed, the propulsion carrying it several feet.

 

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